1. Field of the Invention
This invention relates to an insoluble anode provided with a porous metal substrate. More specifically, it relates to a coating type insoluble lead dioxide anode in which a porous metal is used as a substrate and which is for oxygen generation by electrolysis of water in an aqueous acidic solution.
2. Description of the Prior Art
As anodes for oxygen generation by electrolysis of water, there have hitherto been used in high proportions carbon electrodes as well as metal electrodes in which a metal substrate is plated with platinum or the like. These anodes have various defects in respects of oxygen resistance, corrosion resistance, economization, etc., and metal anodes which can exhibit excellent corrosion resistance particularly in acidic electrolytic solutions have not yet sufficiently been put into practice. Under such circumstances, attention has recently been drawn to lead anode as an insoluble anode having excellent corrosion resistance, dimension stability and electroconductivity even in acidic electrolyte solutions. However, it is very difficult at present to put the lead dioxide anode into practice because lead dioxide does not posses at all malleability and toughness like metals and is very brittle though lead dioxide has excellent corrosion resistance. In order to overcome these inherent defects of lead dioxide and bring the lead dioxide anode into practice, many attempts have been directed to the so-called coating type lead dioxide anodes in which a lead dioxide electrode layer is attached to an expanded metal-like, electroconductive metal substrate (e.g. expanded metal-like titanium) through an intermediate coating layer formed by platinum plating or silver plating for keeping good electrical contact while avoiding direct contact between lead dioxide and the substrate so as to cover the expanded metal-like substrate completely therewith. In this type of lead dioxide anode, the brittleness of lead dioxide itself is reduced to some extent and an electrode body per se can be formed; however its physical strength is extremely weak and the lead dioxide electrode layer is easily peeled off from the substrate even by a slight shock or bending stress. Therefore, said lead dioxide anode cannot be used as a large scale industrial electrode of filter press type and is used practically only in a small scale electrode of so-called dipping type. In order to remedy such a great disadvantage in practice that the lead dioxide electrode layer tends to peel from the substrate, such a proposal has been made that a lead dioxide electrode layer is attached to a porous substrate as if the electrode took root to the substrate. This porous substrate is roughly classified into electroconductive substrates such as graphite, porous sintered titanium and the like, and non-electroconductive substrates such as ceramics, sintered resins, and the like. Anodes using graphite as a porous electroconductive substrate are proposed in Japanese Patent Publication No. 24,313/67, Japanese Patent Application Kokai (Laid-Open) No. 18,283/70, etc. In these anodes, the peeling of the lead dioxide electrode layer from the substrate could be prevented but the substrate per se is far more brittle and cracks more easily than metallic substrates, and accordingly, the primary purpose as anode cannot be sufficiently achieved. In order to improve not only the resistance of the lead dioxide electrode layer to peeling from the substrate but also the resistance of the substrate per se to cracking, a porous sintered metal has been taken into consideration as an anode substrate, and porous sintered titanium is used as the substrate in view of the corrosion resistance thereof (see Japanese Patent Application Kokai (Laid-Open) No. 78374/79). In this case, a porous sintered titanium substrate is dipped in molten lead to fill the inner parts and openings of pores with lead and the surface of lead present at the pore opening is then anodized into lead dioxide. This anode has two problems. Firstly, not the whole outer surface of the electrode body but only lead dioxide of the pore openings function as an electrode and accordingly the actual electricity-flowing area is extremely small. Secondly, inside the pores, the lead portions contact directly with the lead dioxide portions near the surfaces, whereby lead is gradually oxidized by lead dioxide and non-electroconductive PbO layers are formed at the interfaces of the two portions, whereby it is made essentially impossible to keep good electrical contact over a long perior of time between the porous sintered titanium substrate and the lead dioxide electrode layer.
Ceramics are used as non-electroconductive porous substrates in Japanese Patent Publication No. 28743/77. This type of electrode has the practical disadvantage that physical strengths of ceramics per se are weaker than those of porous metal substrates and large flat plates of ceramics are difficult to obtain. It is also necessary to fix the terminal to the surface of the lead dioxide electrode layer through a metal plate, and this terminal portion is gradually corroded with an electrolytic solution which migrates in the dioxide layer by a capillary phenomenon to the terminal and eventually it becomes impossible for electricity to flow at the corroded portions. This is the essential disadvantage of said ceramic type electrode.
As mentioned above, there have never been any practical lead dioxide anode by which all the problems encountered in its practical application have been solved such as corrosion resistance and physical strengths of anode per se and corrosion resistance of terminal portions.